When a moving body site is imaged by an X-ray CT apparatus, an artifact caused by movement occurs in an obtained tomogram. In order to reduce this artifact, a biomedical sensor such as an electrocardiograph, a respiration sensor or the like is generally used and also a measurement based on equipment for converting a physiological periodic motion to an electrical signal is executed in combination. Scanning and image reconstruction are executed by using the thus-obtained electrical signal. Particularly, a reconstructing method targeting a heart is called as an electrocardiographic-synchronous reconstructing method. According to this method, an electrical signal measured by an electrocardiograph is added to scanning data and collected, and image reconstruction is executed on the basis of obtained electrocardiographic information, whereby a cardiac tomography at any cardiac time phase can be obtained.
For example, according to a mechanism of Patent Document 1, scanning data which are different in scan or view at the same cardiac time phase (hereinafter referred to as “divisional scanning data”) are collected from plural heartbeats by using R-wave of an electrocardiographic waveform as a benchmark. The thus-collected scanning data are combined with one another to reconstruct an image, whereby time resolution can be enhanced.
However, in the divisional type reconstruction based on the mechanism of the Patent Document 1 or the like, the collection pattern of the divisional scanning data varies in accordance with a cardiac rate and a scanning condition, and thus the time resolution varies. The time resolution varies depending on the cardiac rate, the scan speed and the bed moving speed. For example, as the bed moving speed is smaller, the number of collectable divisional scanning data is larger and thus the time resolution is higher. As described above, the time resolution of the divisional type reconstruction is affected by the cardiac rate of a patient and the scanning condition. In an actual scanning operation, a scanning condition is required to be determined in consideration of a breadth-holding time and an exposed dose so as to obtain image quality necessary to make a diagnosis.
For example, there is known a method of determining a scanning condition by using the number of divisional scanning data to be collected as an index to create one image (hereinafter referred to as “first method”). According to the first method, an optimum scanning condition is selected to collect divisional scanning data whose number is specified by an operator. In general, as the number of divisional scanning data to be collected is increased, higher time resolution can be obtained. However, it is necessary to reduce the bed moving speed, so that the exposed dose increases. Conversely, as the number of divisional scanning data is reduced, the bed moving speed can be set to a higher value, so that the exposed dose is reduced, however, the time resolution is lowered.
Furthermore, there is also known a method of determining a scanning condition by using the bed moving speed as an index (hereinafter referred to as “second method”), for example. According to the second method, scanning is executed at a bed moving speed specified by an operator, and image reconstruction is performed by using collectable divisional scanning data. In general, as the bed moving speed is lower, a larger amount of divisional projection data can be collected and thus the time resolution is higher. Conversely, as the bed moving speed is higher, collectable divisional projection data is less and thus the time resolution is lowered.